The present invention relates to a data transmission system composed of a network of plural transmission units connected to a communication medium, in which the right of communication is transferred by a communication right transfer command bearing a destination address.
The "token passing method" is known as a communication control process in a so-called local area network (LAN) in which plural business machines are connected through a simple inexpensive communication channel. In this method, control data or a communication right transfer command, called a token and representing the right of transmission, is circulated in the network, and a transmission unit (hereinafter called a "node") receiving said token, or a node designated by said token alone acquires the right of transmission. Each node starts transmission upon receiving the token, and transfers the token to a downstream node when the transmission is completed.
In the token passing method in a bus network, each node constituting the network is given a specific address number, and a destination address is added to the token when it is transferred to a downstream node. Thus each node acquires the right of transmission only upon receiving a token having a destination address the same as the address number of that unit.
Consequently, if plural nodes in the network have the same address number, the following drawbacks will result.
In such a situation, the token is passed to plural nodes simultaneously, so that said plural nodes simultaneously initiate data transmission, thus resulting in data "collision" on the communication channel.
Such phenomenon leads to a temporary breakdown of the function of the network.
Such situation arises from an error in the address designation when the nodes are newly added or when the structure of the network is altered, but is often found only from a failure in the communication, thus undesirably deteriorating the working efficiency of the system.